A switched reluctance motor (hereafter abbreviated as SR motor) includes a rotor having poles which are generally constructed as salient poles, and a stator having poles which project radially inward. The rotor comprises an iron core which is simply formed by a lamination of iron sheets, while the stator includes a concentrated winding coil for each pole. Each pole on the stator acts as an electromagnet, and the magnetic force from the stator attracts the individual poles of the rotor to cause the rotor to rotate, thus operating the SR motor. Accordingly, the rotor can be rotated in any desired direction by sequentially switching the energization of the coils disposed around the respective poles of the stator in accordance with the rotational position of each pole on the rotor. A prior art for such SR motor can be exemplified by Japanese Laid-Open Patent Application No. 298,940/1989, for example.
SR motor has advantages that it is simple in construction, is robust mechanically and is capable of operation under elevated temperatures. However, as a matter of practice, it is not utilized in actuality. One reason herefor resides in the generation of acoustic noises of a high level during its rotation.
Since the energization for each of the poles on the stator is turned on or off when a respective pole on the rotor assumes a particular rotational position, it will be seen that the magnitide of the magnetic attraction which is applied to the rotor rapidly changes upon such switching. This causes mechanical oscillations of relatively high levels to be produced within the rotor and the stator, giving rise to the generation of acoustical noises.
In Japanese Laid-Open Patent Application No. 298,940/1989 cited above, a rotational position signal having a slow rising and falling edge is produced and is used to provide a slow current rise when a particular coil is to be energized and to provide a slow current fall when the coil is to be deenergized. In this manner, it is possible to suppress the generation of oscillations and acoustical noises from the SR motor.
However, because of the use of such rotational position signal, the effect of suppressing the acoustical noises is reduced for a high speed rotation where the current rises or falls more rapidly when the coil is to be energized or deenergized. In addition, during a low speed rotation, where the current rises or falls slowly when the coil is to be energized or deenergized, it follows that a sufficient current flow cannot be supplied as required to produce a torque of an increased magnitude, resulting in a disadvantage that an efficient rotation of the rotor is prevented. In particular, as the rotational speed of the rotor increases, the duration during which the coil is energized per switching is reduced to reduce the magnitude of the current flow, whereby the magnitude of rotating torque generated is reduced. In addition, this suffers from a degraded. efficiency or a failure to provide required torque unless the timing of turning on or off the coils is changed in accordance with the number of revolutions or the magnitude of torque required.